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31EDUCATION EXHIBIT

Eva Castañer, MD • Xavier Gallardo, MD • Eva Ballesteros, MD • Marta Andreu, MD • Yolanda Pallardó, MD • Josep Maria Mata, MD, PhD Lluis Riera, MD

Chronic pulmonary thromboembolism is mainly a consequence of incomplete resolution of pulmonary thromboembolism. Increased vascular resistance due to obstruction of the vascular bed leads to pulmonary hypertension. Chronic thromboembolic pulmonary hy-pertension is clearly more common than previously was thought, and misdiagnosis is common because patients often present with non-specific symptoms related to pulmonary hypertension. Computed tomography (CT) is a useful alternative to conventional angiography not only for diagnosing chronic pulmonary thromboembolism but also for determining which cases are treatable with surgery and confirming technical success postoperatively. The vascular CT signs include direct pulmonary artery signs (complete obstruction, partial obstruction, ec-centric thrombus, calcified thrombus, bands, webs, poststenotic dilata-tion), signs related to pulmonary hypertension (enlargement of main pulmonary arteries, atherosclerotic calcification, tortuous vessels, right ventricular enlargement, hypertrophy), and signs of systemic collateral supply (enlargement of bronchial and nonbronchial systemic arteries). The parenchymal signs include scars, a mosaic perfusion pattern, focal ground-glass opacities, and bronchial anomalies. The presence of one or more of these radiologic signs arouses suspicion and allows diagno-sis of this entity. Early recognition of chronic pulmonary thromboem-bolism may help improve the outcome, since the condition is poten-tially curable with pulmonary thromboendarterectomy.©RSNA, 2009 • radiographics.rsnajnls.org

CT Diagnosis of Chronic Pulmonary Thromboembolism1

LEARNING OBJECTIVES FOR TEST 1After reading this article and taking the test, the reader

will be able to:

Describe the risk ■

factors, clinical manifestations, and pathophysiology of chronic pulmonary thromboembolism.

Define appropriate ■

CT techniques for detecting and evalu-ating pulmonary thromboembolic disease.

Recognize the CT ■

features of chronic pulmonary throm-boembolism, espe-cially those that aid selection of patients for surgical treat-ment.

RadioGraphics 2009; 29:31–53 • Published online 10.1148/rg.291085061 • Content Codes: 1From the Department of Radiology, UDIAT-Centre Diagnòstic, Institut Universitari Parc Taulí-UAB, Parc Taulí s/n, Sabadell 08208 (Barcelona), Spain (E.C., X.G., E.B., M.A., J.M.M., L.R.); and Department of Radiology, Hospital de la Ribera, Alzira, Spain (Y.P.). Recipient of a Certificate of Merit award for an education exhibit at the 2007 RSNA Annual Meeting. Received March 19, 2008; revision requested May 12 and received July 10; accepted August 4. All authors have no financial relationships to disclose. Address correspondence to E.C. (e-mail: [email protected]).

See the commentary by Klein following this article.

©RSNA, 2009

CME FEATURESee accompanying

test at http://www.rsna.org

/education/rg_cme.html

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32 January-February 2009 radiographics.rsnajnls.org

IntroductionMost pulmonary thromboemboli resolve without sequelae. For reasons that are still unclear, in a small percentage of patients, the thromboemboli do not resolve but rather form endothelialized fibrotic obstructions of the pulmonary vascular bed. The result is vascular stenosis, which may lead to severe pulmonary hypertension and cor pulmonale (1). The bronchial circulation re-sponds to decreased pulmonary flow and ische-mia with enlargement and hypertrophy (2); in addition, transpleural systemic collateral vessels (eg, intercostal arteries) may develop (3).

Clinical symptoms in patients with chronic pulmonary thromboembolism are nonspecific and are related to the development of pulmonary hypertension. Symptoms worsen as the right ventricular functional capacity deteriorates (4). Chronic thromboembolic pulmonary hyperten-sion often is identified during the diagnostic work-up in patients with unexplained pulmonary hypertension, and radiologists must be aware of its radiologic manifestations because it is a treat-able cause of pulmonary hypertension in some patients.

The prevalence of chronic thromboembolic pulmonary hypertension in the general popula-tion has yet to be accurately determined and may have been significantly underestimated. Recent prospective epidemiologic data indicate an inci-dence of approximately 4% after acute symptom-atic pulmonary thromboembolism (5,6). Given the growing number of patients undergoing chest computed tomography (CT) who might have experienced a previous episode of pulmonary embolism (either known or unsuspected), incom-pletely resolved emboli are an increasingly com-mon finding on chest CT images.

In this article, we review the risk factors, clini-cal characteristics, and pathogenesis of chronic pulmonary embolism. We describe the optimal technique for CT angiography and the CT di-agnostic criteria for chronic pulmonary throm-boembolism. Finally, we briefly discuss the dif-ferential diagnoses, diagnosis, and treatment of this entity.

Risk Factors and Clinical Manifestations

Women are affected slightly more frequently, and patients with underlying malignant, cardiovascu-

lar, or pulmonary disease are at increased risk of developing this condition (6,7). Other reported risk factors include splenectomy, ventriculoatrial shunts, chronic inflammatory disorders, and myeloproliferative syndromes (4,8). Ethnic dif-ferences in the clinical characteristics of chronic thromboembolic pulmonary hypertension (eg, it is more prevalent in Asian patients after an acute episode of pulmonary thromboembolism) are suggestive of the involvement of a genetic factor in the etiology or pathogenesis of the condition (9).

In a recent prospective study (5) in symptom-atic patients with unexplained persistent dyspnea after treatment for acute pulmonary thromboem-bolism, it was observed that multiple episodes of pulmonary embolism, larger perfusion defect, younger age, and idiopathic manifestation of pul-monary thromboembolism were associated with an increased risk of chronic thromboembolic pul-monary hypertension.

Lupus anticoagulant, a prothrombotic factor, is detected in approximately 10% of patients with chronic thromboembolic pulmonary hyper-tension (10), and 20% carry anticardiolipin antibodies, lupus anticoagulant, or both (4). Plasma levels of factor VIII and antiphospholipid antibodies, two thrombophilic factors that are as-sociated with recurrent thrombosis, are elevated in patients with chronic thromboembolic pulmo-nary hypertension (11).

Symptoms are nonspecific and are related to the development of pulmonary hypertension. The extent of vascular obstruction is a major determinant of the severity of pulmonary hyper-tension. In the majority of symptomatic patients, more than 40% of the pulmonary vascular bed is obstructed (10,12). Patients with chronic thromboembolic pulmonary hypertension may be asymptomatic for several years before their presentation with symptoms such as recurrent acute or progressive exertional dyspnea, chronic nonproductive cough, atypical chest pain, tachy-cardia, syncope, and cor pulmonale (7,10,12,13). The clinical deterioration parallels the loss of right ventricular functional capacity. In these patients, pulmonary arterial pressure is elevated, right atrial pressures are high, cardiac output is reduced, and pulmonary capillary wedge pres-sures are normal (12).

PathogenesisThe natural history of pulmonary thromboem-boli in more than 90% of patients includes total

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RG ■ Volume 29 • Number 1 Castañer et al 33

resolution or resolution with minimal residua and restoration of normal pulmonary hemodynamics within 30 days after treatment. Early resolution of pulmonary vascular obstruction occurs by two mechanisms: mechanical fragmentation and en-dogenous fibrinolysis (4).

The pathogenesis of chronic thromboembolism is still unclear; extensive analyses of plasma proteins in patients with chronic thromboembolic pulmonary hypertension have shown no abnor-malities in fibrinolysis. Nevertheless, the majority opinion supports a thromboembolic pathogen-esis, with a possible role for in situ thrombosis as a factor in disease progression (contribution to the persistence of thrombi) rather than initiation (4,6,14,15). In this scenario, the pathologic pro-cess is linked mainly with disturbance of throm-bus resolution (Fig 1). In a small percentage of patients, particularly those with a large perfusion defect or recurrent episodes of thromboembo-lism, the resolution of thrombi is incomplete (4,16). The organization of a thromboembolus with invasion by fibroblasts and capillary buds that adhere to the vascular wall can be regarded as a reparative response. Shrinkage of a throm-boembolic mass allows restoration of a portion of the original lumen. The remaining embolic material is incorporated into the vessel wall and covered over by a thin layer of endothelial cells. The thromboembolic material may lead to ob-struction, stenosis, and subsequent atrophy of the vessel. Some blood flow may be restored by recanalization through the obstructive mass. In

some instances, all that remains of an organized thromboembolus are fibrous cords, which may occur singly, in bands, or may form a weblike network (17).

The hemodynamic basis of continuing pulmo-nary hypertension in these patients is not only the occlusion of pulmonary arteries but also the development of distal arteriolar vasculopathy (in nonobstructed areas as well as in partially or to-tally obstructed arteries) as a result of pulmonary hypertension (15,18).

In the presence of chronic thromboembolic disease, the bronchial and nonbronchial systemic circulation is markedly increased as a result of the development of systemic-to-pulmonary anasto-moses, which help to maintain pulmonary blood flow in the presence of vessel obstruction (19,20).

CT TechniqueChronic pulmonary thromboembolism is often identified during the diagnostic work-up in pa-tients with unexplained pulmonary hypertension. At our institution, most cases of chronic pulmo-nary thromboembolism are discovered at CT pulmonary angiography performed to rule out acute pulmonary thromboembolism. We perform CT pulmonary angiography with a multidetector CT scanner (Sensation 16; Siemens, Erlangen, Germany) (120 kV, 70–120 mAs, 0.5 second scanning time, 0.75 mm detector width, pitch of 1.5). Images are reconstructed with a 1-mm section thickness at a 0.7-mm interval. Patients receive 100 mL of contrast material (iopromide, Ultravist 300; Schering, Berlin, Germany) at an injection rate of 4 mL/sec. When chronic pulmo-nary embolism is suspected, we modify the CT pulmonary angiography protocol: The intrave-nous administration of the contrast material bo-lus is timed so that both the pulmonary and the systemic circulation are opacified. The bronchial circulation, which usually originates from the de-scending aorta, is markedly increased in patients with chronic pulmonary thromboembolism, and the enhancement of bronchial vessels may aid in the differential diagnosis (20). Bolus timing also allows assessment of all cardiac chambers. The desired opacification of the pulmonary and systemic circulation can be achieved by using a longer delay from contrast material injection to image acquisition; we use a higher trigger thresh-old, 200 HU (21) (our threshold for evaluation of acute pulmonary embolism is 120 HU) with a

Figure 1. Diagram shows the various possible results of disturbed resolution of a thrombus: vascular stenosis, retraction with total obstruc-tion, retraction with partial obstruction, recanali-zation, or residual fibrous cords (web or bands).

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of the contrast material bolus, a feature that has been described as a “pouch defect” (24). At CT, this feature is difficult to see, and the additional findings of an abrupt decrease in vessel diameter and absence of contrast material in the vessel seg-ment distal to the total obstruction are easier to identify (Fig 3a) (25). The reduction in vessel di-ameter is persistent and is caused by contraction of the thrombus (26). CT scans viewed at lung window settings depict segmental and subseg-mental vessels that are abnormally small in com-parison with the accompanying bronchi (Fig 3b).

Partial Filling Defects.—An organized throm-bus may cause vessel narrowing, intimal irregu-larities, bands, and webs. Abrupt vessel narrowing is caused by recanalization within a large throm-bus or by stenosis due to an organized thrombus that lines the arterial wall (24). In the presence of recanalization, contrast material is seen flow-ing through thickened and often smaller arteries (Fig 2). The organized thrombus runs parallel to the arterial lumen and appears as a thickening of the artery wall (24,27), sometimes producing an irregular contour of the intimal surface (Fig 4a). A chronic thrombus in an artery with a course

circular region of interest centered on the main pulmonary artery.

We perform scanning in the caudal-cranial direction because most pulmonary emboli are found in the lower lung lobes (17) and, if the patient is unable to sustain breath holding throughout image acquisition, the lower lobes are imaged in the initial seconds of the breath hold. Because some signs of chronic thromboembolism (eg, bands) may be overlooked with the high-contrast mediastinal window settings, we view the images by using three different gray scales for in-terpretation: a lung window (window width, 1500 HU; window level, -600 HU), a mediastinal window (window width, 350 HU; window level, 40 HU), and a pulmonary thromboembolism–specific window (window width, 700 HU; win-dow level, 100 HU) (22).

Multiplanar reformatted images and maxi-mum intensity projection images that provide longitudinal views of vessels may help clarify confusing or questionable findings and may bet-ter depict obstructions, stenoses, and flattened peripheral thrombotic material (Fig 2) that other-wise might be overlooked (23).

CT Features of Chronic Pulmonary Thromboembolism

We classify the CT features of chronic pulmonary thromboembolism as vascular signs or paren-chymal signs. The vascular signs include direct pulmonary artery signs (results of thrombus or-ganization), signs due to pulmonary hypertension (results of the sustained increase in pulmonary vascular resistance), and signs due to systemic collateral supply (results of decreased pulmo-nary artery flow). The parenchymal signs include scars, a mosaic perfusion pattern, focal ground-glass opacities, and bronchial dilatation. The vascular and parenchymal signs are described in greater detail in the next sections.

Vascular Signs

Pulmonary Arterial SignsThe signs seen in the pulmonary artery at CT are similar to those seen at conventional angiography.

Complete Obstruction.—At angiography, complete vessel cutoff results in a convex margin

Figure 2. Chronic pulmonary thromboembolism in a 47-year-old man. Coronal 10-mm-thick maximum intensity projection image from CT shows a flattened eccentric thrombus in the right pulmonary artery (black arrows)—a feature not well depicted on axial images—and abrupt stenosis and recanalization of the right interlobar artery (arrowheads). Bronchial arteries in the mediastinum and around the stenosed vessels (white arrows) appear abnormally enlarged.


dilatation or aneurysm may be observed (24) (Figs 4a, 5).

that is transverse to the scanning plane has the appearance of a peripheral, crescent-shaped in-traluminal defect that forms obtuse angles with the vessel wall (25–27) (Fig 4b). Poststenotic

Figure 3. Chronic pulmonary thromboembolism in a 65-year-old man with a history of multiple episodes of acute pulmonary thromboembolism. (a) Axial contrast-enhanced CT scan shows complete occlusion and marked reduc-tion in size of the right lower lobe pulmonary artery (arrows) in comparison with the left lower lobe pulmonary artery, which contains a residual intraluminal band (arrowhead). (b) Axial CT scan (lung window) shows segmental and subsegmental vessels in the right lower lobe that are abnormally small compared with their accompanying bron-chi. Peripheral nodular opacities (arrowheads) in the right lower and right middle lobes are secondary to previous infarction.

Figure 4. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmonary thromboembolism. (a) Axial contrast-enhanced CT scan shows bilateral eccen-tric chronic thrombi producing irregular contours of the intimal surface of both main pulmonary arteries (arrows) and poststenotic dilatation (arrowheads) in the posterior segmental artery of the right upper lobe. (b) Axial contrast-enhanced CT scan shows an eccentrically located thrombus with a broad base forming obtuse angles with the vessel wall in the left lower lobe pulmonary ar-tery (arrows).


has a length of 0.3–2 cm and width of 0.1–0.3 cm. It is often oriented in the direction of blood flow, along the long axis of the vessel (28). A web consists of multiple bands that have branches

A band is defined as a linear structure that is anchored at both ends to the vessel wall and has a free, unattached midportion. A band generally

Figure 5. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmonary thromboembolism (same patient as in Fig 4). (a) Oblique coronal 30-mm-thick maximum intensity projection CT image shows aneurysms in the posterior segment of the right upper lobe pulmonary artery (*) and aneurysmal dila-tation of the right lower lobe pulmonary artery (arrows) distal to a band (arrowhead). (b) Oblique coronal 10-mm-thick maximum intensity projection CT image provides a closer view of the band (arrowhead) and the poststenotic dilatation. The marked increase in diameter and the tortuosity of the pulmonary arteries (arrows in b) are indicative of pulmonary hypertension.

Figure 6. Residual band from a pulmonary thrombus in an 83-year-old woman with dyspnea. (a) Axial contrast-enhanced CT image shows a linear structure anchored to the vessel wall in the left lower lobe pulmonary artery (arrow). (b) Coronal 10-mm-thick maximum intensity projection CT image shows the attachment of the band to the vessel wall in more detail (arrows).


pulmonary hypertension, regardless of the cause; such enlargement is a common finding in patients with chronic thromboembolic pulmonary hyper-tension (Figs 4a, 5, 7) (30). The CT diameter of the main pulmonary artery is measured in the scanning plane of its bifurcation, at a right angle to its long axis and just lateral to the ascending aorta (Fig 8). When the ratio of the diameter of the main pulmonary artery to the diameter of the aorta measured on CT scans is greater than 1:1, there is a strong correlation with elevated pulmonary artery pressure, especially in patients younger than 50 years (31) (Fig 8). In contrast to the symmetric pulmonary enlargement typically seen in nonthromboembolic pulmonary hyper-tension, central pulmonary arteries in patients with chronic thromboembolic pulmonary hyper-tension often are asymmetric in size (32) (Figs 4a, 8). The walls of the pulmonary arteries may show atherosclerotic calcification (13) (Fig 8). Tortuous pulmonary vessels have been described in patients with pulmonary hypertension and are seen also in patients with chronic thromboembo-lic pulmonary hypertension (33) (Fig 5b).

forming a network. At CT angiography, bands and webs are depicted as thin lines surrounded by contrast material (Figs 3a, 5, 6). These fea-tures most frequently are found in lobar or seg-mental arteries and rarely are seen in the main pulmonary artery (24,28).

Calcifications within chronic thrombi are seen in a small number of patients. On contrast-enhanced CT images with the usual mediastinal window settings, calcified thrombi may be ob-scured by surrounding contrast material. Selec-tion of a wider window setting or creation of maximum intensity projections are helpful for visualizing calcification (25) (Fig 7). Calcified thrombi in subsegmental arteries are often indis-tinguishable from calcified lung nodules. How-ever, their tubular shape and location at the site of arterial branching may aid in their differentia-tion (25).

Signs of Pulmonary HypertensionIncreased vascular resistance due to the ob-structed vascular bed leads to dilatation of the central pulmonary arteries. Enlargement of the main pulmonary artery to a diameter of more than 29 mm (13,29) may occur in the presence of

Figure 7. Chronic pulmonary thromboembolism in an 86-year-old woman. Axial contrast-enhanced CT scan viewed with a wide window setting (width, 1100 HU; level, 100 HU) to facilitate calcium detection shows a partially calcified thrombus in the right pul-monary artery.

Figure 8. Chronic pulmonary thromboembolism and pulmonary hypertension in a 42-year-old man. Axial contrast-enhanced CT scan shows an enlarged pulmonary trunk with a maximum diameter of 39 mm (line) near its bifurcation and asymmetric enlargement of the right pulmonary artery secondary to an exten-sive thrombus (*). Atherosclerotic calcification of the left pulmonary artery also is visible (arrows).


their widest points, in diastole, between the in-ner surface of the free wall and the surface of the interventricular septum (Fig 9a). The diastolic maxima of the right and left ventricles may be reached at slightly different levels. Right ventricu-lar enlargement may be accompanied by dilata-tion of the tricuspid valve annulus and resultant tricuspid valve regurgitation (Fig 9b).

Patients with severe pulmonary hypertension may present with mild pericardial thickening or a small pericardial effusion (35). The presence of pericardial effusion implies a worse prognosis (36). Patients with chronic thromboembolic pul-monary hypertension may have enlarged lymph nodes (37). At histologic examination of these enlarged nodes, a vascular transformation of the lymph node sinus may be seen, often in associa-tion with sclerosis of varying degrees. Similar histologic features may be observed also in lymph nodes from patients with pulmonary hyperten-sion due to other causes (25).

Right heart disease is a common and expected finding secondary to pulmonary hypertension: The increased workload borne by the right heart results in right ventricular enlargement and hypertrophy (right ventricular myocardial thick-ness greater than 4 mm) (25) (Fig 9a). Over time, right ventricular function deteriorates, even in the absence of recurrent embolism, presum-ably because of the development of hypertensive vascular lesions in the nonobstructed pulmonary artery bed and of vasculopathy in vessels distal to obstructed arteries (18). Dilatation of the right ventricle is considered present when the ratio of the diameter of the right ventricle to that of the left ventricle is greater than 1:1 and there is bow-ing of the interventricular septum toward the left ventricle (34). At CT, these signs can be evalu-ated even without electrocardiographic gating. The minor axes of the right and left ventricular chambers can be measured in the axial plane at

Figure 9. Right heart abnormalities secondary to chronic thromboembolic pulmonary hypertension in a 47-year-old man (same patient as in Fig 2). (a) Axial contrast-enhanced CT scan shows dilatation of the right ventricle (RV), with a ratio of more than 1:1 between the right and left ventricle (LV) diam-eters (lines); leftward septal bowing (arrowheads); thickening of the free right ventricular wall (arrows); and dilatation of the right atrium (RA). (b) Axial contrast-enhanced CT scan at a lower level shows opacification of the inferior vena cava and suprahepatic veins because of retrograde flow of contrast ma-terial, which is often seen in patients with elevated right atrial and right ventricular pressures.


nary arteries downstream, systemic-to-pulmonary arterial anastomoses develop beyond the level of obstruction (41). The bronchial arteries usually arise from the descending aorta at the level of the carina. Abnormal dilatation of the proximal por-tion of the bronchial arteries (diameter of more than 2 mm) and arterial tortuosity (Figs 2, 10) are CT findings indicative of bronchial artery hypervascularization.

In a recent study (20), abnormally enlarged bronchial and nonbronchial systemic arteries were found more frequently in patients with chronic thromboembolic pulmonary hypertension (73%) than in patients with idiopathic pulmonary hyper-tension (14%); these findings could help distin-guish between these two entities. In this study, the most frequently depicted abnormal nonbronchial systemic arteries were the inferior phrenic, inter-costal (Fig 11), and internal mammary arteries.

Collateral Systemic SupplyBronchial arterial flow increases in response to chronic obstruction of the pulmonary arteries in patients with chronic thromboembolic pulmo-nary hypertension. In addition, transpleural sys-temic collateral vessels (eg, intercostal arteries) may develop (20,38). Normally, the bronchial ar-teries only supply nutrition to the bronchi and do not take part in gas exchange. However, in patho-logic conditions that diminish pulmonary artery circulation, flow through the bronchial arteries increases, and they participate in blood oxygen-ation (2). Systemic hypervascularization is a non-specific response to stimuli such as reduced pul-monary artery flow, hypoxemia, fibrosis, chronic inflammation, and chronic infection. The normal bronchial arterial blood flow is of the order of 1%–2% of the cardiac output. In patients with chronic thromboembolic pulmonary hyperten-sion, bronchial flow may represent almost 30% of the systemic blood flow (39,40). To fill pulmo-

Figure 10. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmo-nary thromboembolism (same patient as in Fig 4). (a) Axial contrast-enhanced CT scan shows enlargement of the proximal portion of the right bronchial artery (arrowhead) and aneurysmal dilatation of an upper lobe artery (*). (b) Oblique coronal 20-mm-thick maximum intensity projection CT image better depicts the course of the right bronchial artery (arrowheads) and shows a large eccentric thrombus in the main and left lower lobe pulmonary arteries (*). The bronchial circulation appears as fine hyperattenuating lines inside the thrombus.


age and take on the more linear shape of a paren-chymal band (Fig 14) (43). Parenchymal scars often occur in multiples, generally are found in the lower lobes, and often are accompanied by pleural thickening (20).

Acute pulmonary embolism does not appear to cause dilatation of the bronchial arteries; in pa-tients in whom the distinction between acute and chronic or recurrent pulmonary embolism at CT angiography is unclear, the presence of dilated bronchial arteries should favor the diagnosis of chronic or recurrent pulmonary embolism (38).

Another important finding is that dilated bronchial arteries are positively correlated with a lower mortality rate after pulmonary thrombo-endarterectomy (41). Development of systemic hypervascularization may also be responsible for hemoptysis in these patients (42).

Parenchymal SignsScars from prior pulmonary infarctions are commonly depicted in areas of decreased lung attenuation on CT scans obtained in patients with chronic thromboembolic pulmonary hyper-tension (13). These scars may appear as paren-chymal bands, wedge-shaped opacities (Fig 12), peripheral nodules, cavities (Fig 3b), or irregular peripheral linear opacities (Fig 13) (1,32,43). The appearance most suggestive of scar tissue from infarction is a wedge-shaped pleura-based opacity; however, an infarct may constrict with

Figure 11. Chronic pulmonary thromboembolism in a 47-year-old man with multiple episodes of acute pul-monary thromboembolism. (a) Coronal 30-mm-thick maximum intensity projection CT image shows marked enlargement of branches of the right and left inferior phrenic arteries (straight arrows), right and left bronchial arteries (arrowheads), and an intercostal artery (curved arrow). (b) Coronal 30-mm-thick maximum intensity projection image shows enlargement of intercostal arteries on the right side compared with those on the left.

Figure 12. Chronic pulmonary thromboembolism. CT scan (lung window) obtained in an 85-year-old woman 2 years after an episode of massive acute thromboembolism shows a subpleural wedge-shaped area of consolidation (arrow) in a region of decreased attenuation, a feature indicative of pulmonary infarc-tion. The region did not enhance after contrast mate-rial was administered.


areas, and increased attenuation has been related to the redistribution of blood flow to the patent arterial bed (12,32). These assumptions can be confirmed by several findings: larger vessels in re-gions of increased attenuation, stronger enhance-ment of the hyperattenuating areas after contrast material administration (1,44), and correlation between areas of low attenuation on CT images and areas of hypoperfusion depicted at single photon emission computed tomography (32).

Mosaic lung attenuation is nonspecific and is seen more often in patients with pulmonary hypertension due to vascular disease than in those with pulmonary hypertension due to car-diac or lung disease. Mosaic perfusion is seen much more commonly in patients with chronic thromboembolic pulmonary hypertension than in patients with idiopathic pulmonary hypertension (45).

Systemic perfusion of the peripheral pulmo-nary arterial bed accounts for the presence of isolated focal areas of ground-glass attenuation (33,46) (Fig 15).

Arakawa et al (44) found direct evidence of airway obstruction—air trapping on expiratory CT images—in patients with chronic pulmonary thromboembolism. Air trapping is commonly

A mosaic pattern of perfusion also is seen at CT in the presence of chronic thromboembolic pulmonary hypertension. This pattern appears as sharply demarcated regions of decreased and increased attenuation because of irregular perfu-sion (Figs 12, 14). Low attenuation is due either to hypoperfusion in areas distal to occluded ves-sels or to distal vasculopathy in nonoccluded

Figure 13. Chronic pulmonary thromboembolism. CT scan (lung window) obtained in a 48-year-old man shows multiple peripheral linear opacities in both lower lung lobes, features that represent old infarcts.

Figure 14. Chronic pulmonary thromboembolism in a 65-year-old man. CT scan (lung window) shows a mosaic perfusion pattern with marked regional variations in attenuation of the lung parenchyma and disparity in the size of the segmental vessels, with larger-diameter vessels in regions of increased attenu-ation (arrows). A peripheral parenchymal band or scar (arrowhead) from infarction also is depicted.

Figure 15. Chronic pulmonary thromboembolism in an 80-year-old woman with a history of acute pulmo-nary thromboembolism (same patient as in Figs 4 and 10). Coronal 20-mm-thick maximum intensity projec-tion CT image shows foci of ground-glass attenuation in the right upper lobe (arrows) secondary to systemic perfusion in peripheral areas, as well as bilateral en-largement of the bronchial arteries (arrowheads).


Cylindrical bronchial airway dilatation is seen in two thirds of patients with chronic thromboem-bolic pulmonary hypertension (47). It occurs at the level of segmental and subsegmental bron-chi, adjacent to severely stenosed or completely obstructed and retracted pulmonary arteries (Fig 17). Two different hypotheses might explain this phenomenon. First, findings by Wetzel et al (48) in an investigation of the airway response to hypoxia in a pig model are suggestive of hypoxic bronchodilatation. Another hypothesis involves

seen in areas of hypoperfusion due to chronic embolism (Fig 16). Arakawa and colleagues found significant associations between the ap-pearance of air trapping and the presence of a proximal arterial stenosis or clot and between the extent of air trapping and the degree of impair-ment of pulmonary function of the small airways in these patients (44).

Figure 16. Chronic pulmonary thromboembolism in an 82-year-old woman. (a) Axial inspiratory CT scan (lung window) shows chronic thromboembolism of the left lower lobe pulmonary arteries. Note the mosaic perfusion pattern and the diminished size of vessels in this lobe compared with the right lower lobe. (b) Axial expiratory CT scan (lung window) at the same level as a shows evidence of air trapping in areas of lower at-tenuation. Air trapping is not specific to airway disease and may be a sign of chronic thromboembolism.

Figure 17. Chronic pulmonary thromboembolism in an 82-year-old woman. (a) Axial CT scan (lung win-dow) shows increased bronchial diameters and an absence of normal distal tapering of the segmental and sub-segmental bronchi of the left lower lobe (arrow). Note the small arterial segments (arrowhead) at the lateral border of each dilated bronchus. (b) Axial contrast-enhanced CT scan at a level slightly higher than a shows a marked reduction of arterial perfusion in the left lower lobe.


may mimic pulmonary artery occlusions caused by thrombotic material of embolic origin (14). Enlargement of bronchial and nonbronchial systemic arteries is seen in 73% of patients with chronic pulmonary thromboembolism but in only 14% of patients with idiopathic pulmonary hy-pertension (20).

The combination of mosaic lung attenua-tion and marked regional variation in the size of segmental vessels is seen frequently in patients with chronic thromboembolic pulmonary hy-pertension and is hardly ever seen in those with idiopathic pulmonary hypertension (43,45). The peripheral opacities typically produced by infarcts are rarely seen in patients with idiopathic pulmo-nary hypertension (43).

Differentiation of Acute and Chronic ThromboembolismChronic pulmonary thromboembolism often is discovered during CT pulmonary angiography performed to rule out acute pulmonary throm-boembolism in a patient who presents with dysp-nea. Acute and chronic thromboembolism com-monly coexist.

In cases of acute complete obstruction, the di-ameter of the pulmonary artery may be increased because of impaction of the thrombus by pulsa-tile flow (49) (Fig 18a). Conversely, in chronic thromboembolic disease, the diameter of the ves-sel distal to a complete obstruction is markedly decreased (Fig 18b).

the bronchial circulation, which supplies nutri-ents to the bronchial walls: If the pulmonary arteries are chronically obstructed, there is an increased demand on the bronchial circulation to provide pulmonary parenchymal perfusion, possibly robbing the bronchial walls of nutrients and weakening them, which leads to airway dila-tation (47).

Although parenchymal findings (eg, mosaic at-tenuation with asymmetric artery size, pulmonary infarcts) are nonspecific, in the appropriate clini-cal setting they may be regarded as supportive of the diagnosis of chronic thromboembolic pulmo-nary hypertension.

Differential DiagnosisChronic thromboembolic pulmonary hyper-tension is often misdiagnosed, and the initial thromboembolic event in most patients is asymp-tomatic or overlooked. Both congenital and acquired conditions may cause pulmonary hyper-tension or obstruction of the pulmonary arteries and mimic chronic thromboembolic pulmonary hypertension.

Idiopathic Pulmonary HypertensionBoth chronic thromboembolic pulmonary hy-pertension and idiopathic pulmonary hyperten-sion manifest clinically with exertional dyspnea, pulmonary hypertension, and signs of right heart failure. Although the diagnosis of chronic thromboembolic disease relies on precise vascu-lar features, its differentiation may be difficult because in situ pulmonary artery thrombosis in patients with idiopathic pulmonary hypertension

Figure 18. Evolution of chronic occlusive pulmonary thromboembolism from acute embolism in a 40-year-old man. (a) Axial contrast-enhanced CT scan shows acute embolism in the left lower lobe, with increased arterial diameters (arrows) due to impacted thrombi. (b) Axial contrast-enhanced CT scan obtained at the same level as a, 1 year later, when the patient presented with dyspnea, shows a per-manent reduction in the diameters of the left lower lobe arteries (arrows) because of thrombus organiza-tion and retraction, findings indicative of chronic thromboembolism.


mated as more than 30%) increases pulmonary vascular resistance and leads to acute pulmonary hypertension and, in some cases, right ventricular dysfunction and dilatation (52). However, since pulmonary hypertension is not firmly established in cases of acute obstruction, right ventricular hy-pertrophy has not yet developed.

An acute nonobstructive filling defect may be central or eccentric in location. In acute thromboembolism, a nonobstructive eccentric filling defect forms acute angles with the vessel wall (Fig 19a). Conversely, partially obstructive chronic thromboembolism appears as a periph-eral crescent-shaped defect that forms obtuse angles with the vessel wall (Fig 4b). An acute nonobstructive central defect appears surrounded by contrast-enhanced blood (Fig 19b) (50).

If the distinction between acute and chronic or recurrent pulmonary thromboembolism is unclear, the presence of dilated bronchial arteries supports a diagnosis of recurrent or chronic pul-monary thromboembolism (38).

Wittram et al (51) defined the attenuation values of acute and chronic pulmonary throm-boembolism. The mean attenuation (± standard deviation) in the presence of chronic thromboem-bolism (87 HU ± 30) is significantly higher than that in acute thromboembolism (33 HU ± 15). The higher mean attenuation in the presence of chronic pulmonary thromboembolism is likely related to enhancement of the organizing throm-bus, retraction of the thrombus with its concen-trations of hemoglobin and iron, and, possibly, calcium deposition (Fig 7) (51).

Acute embolic obstruction of a significant amount of the pulmonary circulation (usually esti-

Figure 19. Acute pulmonary thromboembolism in a 58-year-old woman who presented with dyspnea. (a) Axial contrast-enhanced CT scan depicts an eccentric partial filling defect due to a thrombus that forms acute angles with the wall of the left lower lobe pulmonary artery (arrows) (compare with Fig 4b). (b) Axial contrast-enhanced CT scan shows partial filling defects surrounded by contrast material (“railway track” sign) (arrows), features pro-duced by acute thrombi in the left upper lobe artery.

Figure 20. Unilateral proximal interruption of the right pulmonary artery in a 52-year-old woman with progressive dyspnea. Axial contrast-enhanced CT scan shows only the proximal portion of the right pulmo-nary artery (arrowhead), with enlargement of the main and left pulmonary arteries secondary to pulmonary hypertension. No endoluminal or periluminal changes are depicted. (Reprinted, with permission, from refer-ence 54.)


subsegmental arteries and less commonly of the lobar or main pulmonary arteries (56). In the presence of Takayasu arteritis, CT scans may depict concentric inflammatory mural thicken-ing in affected vessels. In addition, findings of wall thickening in the aorta and aortic branches and the absence of intraluminal thrombi in the pulmonary arteries are diagnostic (57). Unilat-eral pulmonary artery occlusion may occur at an advanced stage of the disease (Fig 21a), and late-stage Takayasu arteritis should be considered in cases of chronic pulmonary artery obstruction of unknown origin (56). As in other conditions involving decreased pulmonary flow, collateral vessels may develop (Fig 21).

Primary Sarcoma of the Pulmonary ArteryPrimary sarcoma of the pulmonary artery is rare. Undifferentiated sarcoma and leiomyosarcoma are the types of sarcoma that most frequently affect the pulmonary arteries. The main or proxi-mal pulmonary arteries are most frequently in-volved. The clinical manifestations may mimic those of acute or chronic pulmonary embolism (58). Contrast-enhanced CT scans show the

Proximal Interruption of the Pulmonary ArteryInterruption of the left pulmonary artery is usu-ally associated with a congenital cardiovascular anomaly, most commonly tetralogy of Fallot. In-terruption of the right pulmonary artery is more common and is an isolated finding in most in-stances (53). The pulmonary artery ends blindly at the hilum, and blood is supplied through collateral systemic vessels, mainly bronchial arteries. Unlike chronic pulmonary embolism, proximal interrup-tion of the pulmonary artery is characterized by smooth, abrupt tapering of the pulmonary artery, without endoluminal changes (Fig 20) (54).

A helpful diagnostic clue in most cases of chronic pulmonary thromboembolism is the presence of multiple bilateral arterial abnormali-ties. Occlusion of one main pulmonary artery, mimicking proximal interruption, is rarely seen and has been reported in only 3% of cases (55).

Takayasu ArteritisTakayasu arteritis is an idiopathic arteritis that mainly affects the elastic arteries. It frequently af-fects the aorta and its major branches; pulmonary artery involvement occurs in 50%–80% of patients and is a manifestation of late-stage disease (56).

The most characteristic findings are steno-sis and occlusion, mainly of the segmental and

Figure 21. Late-stage Takayasu arteritis with right pulmonary artery involvement in a 63-year-old woman. (a) Axial contrast-enhanced CT scan shows right pulmonary artery occlusion (straight arrow), enlarged bronchial arteries (curved arrow) in the right hilum, and an enlarged mammary artery (arrowhead). (b) Axial contrast-enhanced CT scan at the level of the supra-aortic trunks shows soft tissue that surrounds the brachiocephalic trunk (curved ar-rows), occlusion of the left carotid artery (straight arrow), poor visibility of vessels in the right lung because of right pulmonary artery involvement, and development of collateral vessels from intercostal arteries (arrowheads). (Reprinted, with permission, from reference 54.)


contrast, multiple segmental perfusion defects that are unmatched by findings on ventilation scintigrams make chronic thromboembolic pul-monary hypertension the most likely diagnosis, although other conditions, including pulmonary veno-occlusive disease, may result in similar find-ings (60). However, V/Q scintigraphy does not al-low determination of the magnitude, location, or proximal extent of disease and thus cannot pre-dict its surgical operability. V/Q scintigraphy also does not help identify other causes of pulmonary hypertension (61).

Right Heart Catheterization and Pulmonary AngiographyCombined right heart catheterization (to de-termine the severity of pulmonary hyperten-sion) and selective pulmonary angiography (to determine whether thromboembolic disease is present) is considered the reference standard for diagnosis of chronic thromboembolic pulmonary hypertension.

At present, right ventricular catheterization is the best method for determining the mean pul-monary artery pressure and pulmonary vascular resistance, essential data for quantification of the disease severity and determination of the postop-erative prognosis (15).

Conventional pulmonary angiography is the traditional cornerstone for evaluation of chronic thromboembolic pulmonary hypertension. It helps confirm the diagnosis and gives an indica-tion of surgical operability (24,33). However, in

tumor as an intraluminal filling defect that re-sembles a thromboembolus. The filling defect frequently spans the entire luminal diameter of the main or proximal pulmonary artery (Fig 22), a finding that is unusual in pulmonary throm-boembolism. Other findings that may be helpful for distinguishing a pulmonary artery sarcoma from pulmonary thromboembolism include ex-tension of the lesion into the lung parenchyma or mediastinum and delayed enhancement at CT angiography (58). Chong et al (59) reported a case of pulmonary artery sarcoma that showed positive uptake of fluorine 18 fluorodeoxyglucose at positron emission tomography integrated with CT; this feature may be helpful in differentiat-ing a pulmonary artery sarcoma from pulmonary thromboembolism.

Bronchial AbnormalitiesBronchial dilatation is a well-known hallmark of chronic obstructive pulmonary disease (COPD). In this setting, mucus-filled dilated bronchi, pul-monary infiltrates, or both are usually present. From a clinical standpoint, bronchiectasis in patients with COPD is related to sputum pro-duction. CT findings of bronchial dilatation in patients without clinical and functional evidence of COPD should arouse suspicion about the pos-sibility of airway involvement in chronic throm-boembolic disease (47) (Fig 17).

Diagnostic EvaluationWhen chronic thromboembolic pulmonary hy-pertension is suspected, an extensive diagnostic work-up is undertaken. Major goals are to deter-mine whether thromboembolic disease is present, quantify the degree of pulmonary hypertension, and identify the cause or contributing factors.

EchocardiographyTransthoracic echocardiography allows diagnosis of pulmonary hypertension by showing the pres-sures in the right atrium and the degree of tri-cuspid regurgitation. Echocardiography also may help exclude other cardiac causes of pulmonary hypertension (eg, cardiac shunts) (15).

Ventilation-Perfusion ScintigraphyRecently, Tunariu et al (60) reported that venti-lation-perfusion (V/Q) scintigraphy has a higher sensitivity than CT pulmonary angiography for detecting chronic thromboembolic pulmonary hypertension. Normal findings at V/Q scintigra-phy practically rule out the presence of chronic thromboembolic pulmonary hypertension. By

Figure 22. Pulmonary artery sarcoma in a 70-year-old man with dyspnea. Axial contrast-enhanced CT scan shows filling defects in the main, left, and right pulmonary arteries and the right interlobar pulmonary artery. The arterial lumina are expanded, and extravas-cular mediastinal invasion is seen. (Reprinted, with permission, from reference 54.)


The primary treatment for chronic throm-boembolic pulmonary hypertension is surgical pulmonary thromboendarterectomy, which leads to a permanent improvement in pulmonary he-modynamics (66). In this surgical procedure, the thrombus and the adjacent medial layer are care-fully dissected. The reported mortality rate ranges from 4% to 14% (67,68).

Pulmonary thromboendarterectomy is consid-ered for symptomatic patients who have hemody-namic or ventilatory impairment at rest or with exercise; it is also considered for patients who have normal or nearly normal pulmonary hemo-dynamics at rest but in whom marked pulmonary hypertension develops during exercise. The loca-tion and extent of the proximal thromboembolic obstruction are the most critical determinants of surgical operability: Occluding thrombi must involve the main, lobar, or proximal segmental arteries (10,67). If the hemodynamic impairment derives mainly from more distal, surgically inac-cessible disease or from the resistance conferred by secondary small-vessel arteriopathy, then pulmonary hypertension will persist postopera-tively and may have adverse short-term and long-term consequences.

Some preoperative CT angiographic features are considered predictive of a good response to pulmonary thromboendarterectomy. For exam-ple, evidence of extensive central vessel disease and limited small-vessel involvement is consid-ered positive (69,70). Dilated bronchial arteries are positively correlated with a lower mortality rate after the surgical procedure (41). Heinrich et al (71) suggested that patients without dilated bronchial arteries have more severe distal vascu-lar disease (either thromboembolism or second-ary small-vessel disease) leading to higher post-operative pulmonary vascular resistance than that in patients with dilated bronchial arteries.

Placement of a filter in the inferior vena cava is recommended before surgery in all patients except those with a clearly defined source of em-boli other than deep veins in the legs. Lifelong anticoagulant therapy is strongly recommended to prevent recurrent thrombosis after pulmonary thromboendarterectomy (14).

ConclusionsChronic thromboembolic pulmonary hyperten-sion is clearly more common than previously was thought. It often has been misdiagnosed because patients present with nonspecific symptoms. Knowledge of the radiologic imaging signs is required to detect and accurately diagnose the

the future, pulmonary angiography probably will be performed only when an adequate surgical roadmap has not been provided by CT and mag-netic resonance (MR) imaging (62).

CT AngiographyCT angiography is a useful alternative to con-ventional angiography not only for diagnosing chronic thromboembolic pulmonary hyperten-sion but also for determining surgical operability and confirming technical success postoperatively (1,27,33). CT angiography is reported to be more sensitive than conventional angiography in depicting the presence of central thrombotic disease (27) and equally sensitive to MR angiog-raphy in depicting the disease at the segmental level. CT angiography is superior to MR angiog-raphy for the depiction of patent subsegmental arteries and intraluminal webs and for the direct demonstration of thrombotic wall thickening (63). CT angiography also may provide evidence pointing toward an alternative diagnosis or a dif-ferent cause of pulmonary hypertension.

MR ImagingAlthough CT is highly sensitive for the detec-tion of proximal and subsegmental thrombi, it does not yield sufficient quantitative information about the severity of functional impairment. MR imaging does enable sufficient characterization of the impairment of function in the right side of the heart (64). In addition, MR imaging permits accurate estimation of flow in the bronchial ar-teries in patients with chronic thromboembolic pulmonary hypertension (41). It also may play an important role in postoperative follow-up. However, MR imaging cannot take the place of conventional angiography and right heart cath-eterization for the preoperative determination of pulmonary vascular resistance and mean pulmo-nary artery pressure.

MR imaging certainly has the potential to play a central role in the diagnosis, differential diagnosis, treatment planning, and assessment of postoperative outcome in patients with chronic thromboembolic pulmonary hypertension (65).

TreatmentIn chronic thromboembolic pulmonary hyperten-sion, the thromboembolic material is endothelial-ized and incorporated into the vessel wall; there-fore—unlike the situation in acute thromboem-bolism—anticoagulation therapy is not effective. Nevertheless, lifelong anticoagulant therapy is recommended to avoid recurrent thromboembo-lism or in situ growth of existing obstructions of pulmonary arteries (14).

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The main objective of cross-sectional imaging in patients with chronic thromboembolic pulmo-nary hypertension (after confirming the diagnosis and differentiating the disease from other causes of pulmonary hypertension) is to correctly assess the technical feasibility of surgery. In this regard, CT and MR angiography represent the future for diagnosis and management of chronic throm-boembolic pulmonary hypertension.

Acknowledgments: The authors thank Ricard Valero, MD, PhD, for his illustration of interrupted thrombus resolution and John Giba for linguistic aid.

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This article meets the criteria for 1.0 credit hour in category 1 of the AMA Physician’s Recognition Award. To obtaincredit, see accompanying test at http://www.rsna.org/education/rg_cme.html.

RG Volume 29 • Volume 1 • January-February 2009 Castañer et al

CT Diagnosis of Chronic Pulmonary Thromboembolism Eva Castañer, MD, et al

Page 32 The prevalence of chronic thromboembolic pulmonary hypertension in the general population has yet to be accurately determined and may have been significantly underestimated. Recent prospective epidemiologic data indicate an incidence of approximately 4% after acute symptomatic pulmonary thromboembolism. Page 32 Symptoms are nonspecific and are related to the development of pulmonary hypertension. The extent of vascular obstruction is a major determinant of the severity of pulmonary hypertension. In the majority of symptomatic patients, more than 40% of the pulmonary vascular bed is obstructed. Page 33 The pathogenesis of chronic thromboembolism is still unclear [...]. The majority opinion supports a thromboembolic pathogenesis, with a possible role for in situ thrombosis as a factor in disease progression (contribution to the persistence of thrombi) rather than initiation. In this scenario, the pathologic process is linked mainly with disturbance of thrombus resolution. In a small percentage of patients, particularly those with a large perfusion defect or recurrent episodes of thromboembolism, the resolution of thrombi is incomplete. The organization of a thromboembolus with invasion by fibroblasts and capillary buds that adhere to the vascular wall can be regarded as a reparative response. Page 47 CT angiography is a useful alternative to conventional angiography not only for diagnosing chronic thromboembolic pulmonary hypertension but also for determining surgical operability and confirming technical success postoperatively. Page 47 The location and extent of the proximal thromboembolic obstruction are the most critical determinants of surgical operability: Occluding thrombi must involve the main, lobar, or proximal segmental arteries.

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